diff --git a/lib/mbedtls-2.26.0/CMakeLists.txt b/lib/mbedtls-2.26.0/CMakeLists.txt
index 86adb002b07..138adfe2d56 100644
--- a/lib/mbedtls-2.26.0/CMakeLists.txt
+++ b/lib/mbedtls-2.26.0/CMakeLists.txt
@@ -173,6 +173,8 @@ string(REGEX MATCH "Clang" CMAKE_COMPILER_IS_CLANG "${CMAKE_C_COMPILER_ID}")
 
 include(CheckCCompilerFlag)
 
+set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c99")
+
 if(CMAKE_COMPILER_IS_GNU)
     # some warnings we want are not available with old GCC versions
     # note: starting with CMake 2.8 we could use CMAKE_C_COMPILER_VERSION